Seriously though, here are the release noteshttp://dev.mysql.com/doc/refman/5.0/en/news-5-0-x.html
There’s too much for me to go over right now between 15 and 22.
I’d recommend we base the decision on:
Is 22 significantly more stable than 15?
Is there something added between 16 and 22 that we may want to rely upon?
Is there a performance improvement that we want to have associate with WP?

In short, If 22 isn’t significantly better than 15, why would we want to make it the minimum? I’d rather force people to bear the MySQL upgrade pain only when they really need to…